Drugs employed to treat behavior disorders also impair behavior. Phenothiazines, for example, can produce disorders of movement and reduce responsiveness to stimuli in the environment. A detailed analysis of these processes can be helpful in three ways. First, it may contribute to a better grasp of which functions are impaired, and how. Second, it may provide information about differential effects of drugs. Third, it may offer new information about the way in which therapeutic effects might be characterized and assessed. The proposed research focuses on the problem of behavioral toxicity from the standpoint of discriminative stimuli. One major set of experiments emphasizes discriminative motor control. These include studies, in both humans and monkeys, of how accurately forces can be applied with and without accessory external stimuli. A second major set of experiments, mainly with pigeons, emphasizes the ability to discriminate the amount of behavior emitted as a function of behavioral history and access to external discriminative stimuli.